1. Field of the Invention
The present invention relates to plasma guns in which the application of a plasma power source in combination with the introduction of a substantially inert gas at a cathode produces a plasma arc and a resulting plasma stream within an anode in which the cathode is partly disposed, and more particularly to cooling systems for the anode.
2. History of the Prior Art
Plasma guns are known in which a plasma power source coupled between an anode and a cathode combines with the introduction of a substantially inert gas in the region of the cathode to produce an arc within a central plasma chamber within the anode and a plasma stream flowing from the anode. The plasma stream may be directed onto a work piece or other target which is typically coupled to the anode by power supplies to provide a transfer arc. The plasma stream may be used to heat the target. Introduction of powdered material such as powdered metals into the central plasma chamber of the anode causes the metallic powder to be carried to and coated on the target. The operation of the plasma gun may be carried out in atmosphere, although for many applications it is preferred that a vacuum source be coupled to a closed chamber for the plasma gun to provide a low pressure environment and a supersonic plasma stream. Such a plasma system is described in U.S. Pat. No. 4,328,257 of Muehlberger et al, which patent issued May 4, 1982, is entitled "System and Method for Plasma Coating", and is commonly assigned with the present application.
During operation of plasma guns, considerable heat is produced within the gun. A substantial amount of heat is generated in the region of the cathode, typically requiring that a cathode cooling system be provided to circulate water or other cooling fluid in the region of the cathode.
An even greater heating problem occurs in the region of the anode where the plasma arc occurs and the resulting plasma stream is formed within the central plasma chamber. Water or other cooling fluid must be circulated within the gun in the region of the anode to provide cooling of the anode. Anodes are typically of rounded or generally cylindrical configuration, and the cooling water or other cooling fluid is typically delivered to one or more annular passages surrounding the anode where the water circulates before being removed from the gun. Heat from the anode is transferred to the contacting water, and the water as so heated is removed from the gun.
Circulating water systems have proven to be reasonable effective in cooling the anodes of plasma guns, particularly in the case of arrangements where the cooling water is forced to undergo at least a partial revolution around the anode before exiting from the gun. However, the problem of maximizing heat transfer from the anode to the cooling water to provide effective anode cooling is magnified in certain situations such as in the case of plasma guns of relatively small size. So called "mini guns" which measure only a few inches in length and width are commonly used to perform plasma spraying in confined areas such as on the insides of pipes. The mechanics of circulating water or other cooling fluid within the confined spaces of the anode cooling systems of such mini guns is such that effective heat transfer is made difficult. Consequently the anode cooling systems in most such mini guns are of limited effectiveness, and operation of the plasma gun must be carefully monitored to detect overheating. Overheating can occur quickly as the small amounts of cooling water within the confined passages of the anode cooling system may slow or otherwise stagnate long enough to reach the boiling point. As the water begins to boil, it both expands and emits gas, so that continued cooling action is greatly impaired. Such condition must be quickly detected and use of he gun terminated to minimize damage to the gun.
Accordingly, it would be desirable to provide an improved anode cooling system for plasma guns. In particular, it would be advantageous to provide an anode cooling system of increased efficiency and effectiveness and which is particularly well suited for difficult cooling situations such as in the case of mini guns.